Trial-selective effects of U50,488H, a κ-opioid receptor agonist, on consummatory successive negative contrast

A series of experiments studied the effects of the κ-opioid receptor agonist U50,488H on consummatory successive negative contrast (cSNC) in rats. In cSNC, previous experience with a 32% sucrose solution leads to greater rejection of 4% sucrose than exclusive experience with 4% sucrose. Experiments 1 and 2 revealed that U50,488H failed to influence cSNC when administered before the first downshifted trial, but either attenuated (1 mg/kg) or enhanced (3 and 10 mg/kg) cSNC when administered before the second downshift trial. Experiment 3 showed that U50,488H administered immediately after the first downshift trial had no effect on cSNC at the 1 mg/kg dose, but tended to increase cSNC at the 3 mg/kg dose. However, Experiment 4 suggested that the apparent enhancement of cSNC after 3 mg/kg U50 administered posttrial 11 may have reflected the development of a conditioned taste aversion. The trial-selective attenuating effect of the low dose may reflect an anxiolytic-like property of U50,488H.